1. Field of the Invention
The present invention generally relates to time division multiplex (TDM) data transmission systems, and more particularly to a device for selectively obtaining access to individual channels of a TDM bit stream without de-multiplexing thereof, principally to allow testing of such channels.
2. Description of the Prior Art
This invention is related to my copending application "Line Access Module," Ser. No. 725,308 filed Sept. 21, 1976, the pertinent content of which is incorporated into this application by reference.
In the operation of digital transmission systems, it is the practice to multiplex large numbers of subscriber data channels into one high-speed digital bit stream and to transmit the bit stream over various transmission media such as cable and radio. Normally, the bit stream may pass through central offices, relay terminals, and transmission equipment with de-multiplexing of the bit stream being necessary. However, to assure the proper operation of the subscriber end equipment and transmission paths, it is necessary to selectively monitor and periodically test subscriber equipment from central offices.
Prior art subscriber channel testing has been limited to points at which physical access to the individual subscriber lines is possible. In this practice, demultiplex equipment is necessary, manual connections of test equipment to each line to be tested is required, and time-consuming research of network and routing records is required to determine the location of the access points.
One version of a line access module (LAM) to provide convenient access to individual subscriber channels of a high speed TDM data transmission line has been disclosed in the above referenced invention. The LAM described operates in an analog mode and therefore requires external regenerative circuitry when in an active line access condition.